GIPC3, a member of G?-Interacting Protein, C-terminus (GIPC) family, is known to be essential for hearing. Eleven mutations in GIPC3, spread throughout its three structural domains, cause inherited autosomal recessive hearing loss. However, the molecular basis for GIPC3 function in the auditory system and the mechanisms by which these human mutations result in hearing loss are unknown. We have recently determined the structure of GIPC3 bound to a prototypical receptor and determined the molecular mechanism of GIPC3 activation and its subsequent binding to MYO6, the unconventional myosin that is expressed in the auditory hair cells and is essential for hearing. These biochemical and structural studies allowed us to predict the effects of known deafness-related mutations in GIPC3 on its activation and MYO6 binding. We have generated two new mouse models, one that is lacking functional GIPC3 and another one with a W301X point mutation in Gipc3 that corresponds to a mutation with the most severe auditory phenotype in humans. This study will explore the central hypothesis that GIPC3 has a dual role in the auditory hair cells. We hypothesize that in stereocilia GIPC3 is involved in shaping mechanotransduction and hair bundle structure through its interactions with CDH23/myosin-VIIa and myosin-XVa, respectively. In the cell body, GIPC3 is essential for apical endocytosis at the pericuticular neckless region due to its interaction with myosin-VI. All these functions are crucial for normal hearing. However, the exact molecular mechanisms that determine the resting tension in the mechanotransduc- tion machinery as well as gradation of stereocilia heights and diameters in the auditory hair cell bundles are still enigmatic. Likewise, very little is known about molecular mechanisms and even the role of pericuticular neckless endocytosis in the hair cell function. The expected outcomes of this study are to uncover (a) the precise mechanism of deafness associated with GIPC3 deficiency and (b) the physiological role of GIPC3 in hair cell functions, especially in mechanotransduction, formation of the hair bundle architecture, and endocytosis at the pericuticular neckless. Deciphering the function of GIPC3 protein and its known mutations is a critical step towards the development of therapies for the treatment and/or prevention of GIPC3-dependent deafness and hearing loss.